System and method for performing high-speed communications over fiber optical networks

ABSTRACT

Processing a received optical signal in an optical communication network includes equalizing a received optical signal to provide an equalized signal, demodulating the equalized signal according to an m-ary modulation format to provide a demodulated signal, decoding the demodulated signal according to an inner code to provide an inner-decoded signal, and decoding the inner-decoded signal according to an outer code. Other aspects include other features such as equalizing an optical channel including storing channel characteristics for the optical channel associated with a client, loading the stored channel characteristics during a waiting period between bursts on the channel, and equalizing a received burst from the client using the loaded channel characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is filed as a 37 C.F.R. 1.53(b) as a continuationclaiming the benefit under 35 U.S.C. § 120 of the U.S. patentapplication Ser. No. 15/884,351, “System and Method for PerformingHigh-Speed Communications over Fiber Optical Networks”, which was filedby the same inventors on Jan. 30, 2018 claiming the benefit under 35U.S.C. § 120 of the U.S. patent application Ser. No. 15/095,137, “Systemand Method for Performing High-Speed Communications over Fiber OpticalNetworks”, which was filed by the same inventors on Apr. 10, 2016, nowU.S. Pat. No. 9,887,795, claiming the benefit under 35 U.S.C. § 120 ofthe U.S. patent application Ser. No. 12/512,968, “System and Method forPerforming High-Speed Communications over Fiber Optical Networks”, whichwas filed by the same inventors on Jul. 30, 2009, now U.S. Pat. No.9,337,948, claiming the benefit under 37 C.F.R. 1.53(b)(2) of U.S.patent application Ser. No. 11/772,187 filed on Jun. 30, 2007, whichclaims benefit of commonly-assigned U.S. patent application Ser. No.10/865,547 filed on Jun. 10, 2004, now U.S. Pat. No. 7,242,868, whichclaims the benefit of U.S. Provisional Application No. 60/477,845 filedJun. 10, 2003, incorporated herein by reference, and U.S. ProvisionalApplication No. 60/480,488 filed Jun. 21, 2003, incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to optical fiber communications generally, andmore specifically to m-ary modulation in optical communication networks.

BACKGROUND OF THE INVENTION

Line coding is a process by which a communication protocol arrangessymbols that represent binary data in a particular pattern fortransmission. Conventional line coding used in fiber opticcommunications includes non-return-to-zero (NRZ), return-to-zero (RZ),and biphase, or Manchester. The binary bit stream derived from theseline codes can be directly modulated onto wavelengths of light generatedby the resonating frequency of a laser. Traditionally direct binarymodulation based transmission offers an advantage with regard to theacceptable signal-to-noise ratio (SNR) at the optical receiver, which isone of the reasons direct binary modulation methods are used in theDatacom Ethernet/IP, Storage Fiber-Channel/FC and Telecom SONET/SDHmarkets for transmission across nonmultiplexed unidirectional fiberlinks.

The performance of a fiber optic network can be measured by the maximumdata throughput rate (or information carrying capacity) and the maximumdistance between source and destination achievable (or reach). ForPassive Optical Networks (PONs) in particular, additional measures ofperformance are the maximum number of Optical Networking Units (ONUs)and/or Optical Networking Terminals (ONTs) possible on a network and theminimum and maximum distance between the Optical Line Terminator (OLT)and an ONU/ONT. These performance metrics are constrained by, amongother things, amplitude degradation and temporal distortions as a resultof light traveling through an optical fiber.

Amplitude degradation is substantially a function of length or distancebetween two end points of an optical fiber. Temporal distortionmechanisms include intramodal (chromatic) dispersion and intermodal(modal) dispersion. Intramodal dispersion is the dominant temporaldispersion on Single-mode fiber (SMF), while intermodal dispersion isdominant on Multi-mode fiber (MMF). Both types of temporal distortionsare measured as functions of frequency or rate of transmission (alsoreferred as line rate of a communication protocol) over distance inMHz·km. Temporal distortions are greater, hence a constraint on networkperformance, with increasing frequency transmission.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention includes a method forprocessing a received optical signal in an optical communicationnetwork, the method including: determining a first set of coefficientsto equalize a portion of an optical signal received over a first opticallink including using a blind equalization method that does not use aknown training sequence to equalize the portion of the optical signal,equalizing the portion of the optical signal using the determinedcoefficients, and demodulating the equalized portion of the opticalsignal according to an m-ary modulation format.

Aspects of the invention may include one or more of the followingfeatures. The method includes determining a second set of coefficientsto equalize a portion of an optical signal received over a secondoptical link. The method includes selecting one of the first or secondset of coefficients based on a source of the portion of optical signalbeing equalized. The portion of the optical signal includes a burstwithin a time slot of the first optical link. The method includesstoring the determined coefficients. The method includes retrieving thestored coefficients for equalizing a second portion of the opticalsignal corresponding to a portion received from a same source asgenerated the first portion of the optical signal. The coefficients areretrieved between signal bursts on the first optical link. The storedcoefficients are retrieved for respective portions of the opticalsignals that correspond to respective signal sources. The first opticallink includes a link in a point-to-multipoint passive optical network.The m-ary modulation format is selected from the group consisting ofquadrature amplitude modulation, quadrature phase shift keying,orthogonal frequency division multiplexing and pulse amplitudemodulation. The method includes demodulating a received first datastream and demodulating a second data stream received in the opticalsignal, and multiplexing the first and second data streams.

In general, in another aspect, the invention includes opticalcommunication system including: a first transceiver coupled by anoptical network to a second transceiver and third transceiver, the firsttransceiver including an equalization block and a modulation block, theequalization block operable to determine a first set of coefficients toequalize a portion of an optical signal received over the opticalnetwork from the second transceiver and a second set of coefficients toequalize a portion of the optical signal received over the opticalnetwork from the third transceiver, the equalization block including ablind equalization routine that does not use a known training sequenceto equalize the portions of the optical signal, the equalization blockoperable to equalize the portions of the optical signal using thedetermined coefficients, and the modulation block operable to demodulateequalized portions of the optical signal according to an m-arymodulation format.

Aspects of the invention may include one or more of the followingfeatures. The optical network includes a first optical link for couplingthe first and second transceiver, and a second optical link for couplingthe first and third transceivers and where the equalization block isoperable to select one of the first or second set of coefficients basedon a source of the portion of optical signal being equalized. Theequalization block is operable to store the first and second sets ofcoefficients for later retrieval and use to equalize portions of theoptical signal. The portion of the optical signal includes a burstwithin a time slot on the optical network. The equalization block isoperable to retrieve the sets of coefficients between signal bursts onthe optical network. The optical network includes a link in apoint-to-multipoint passive optical network. The m-ary modulation formatis selected from the group consisting of quadrature amplitudemodulation, quadrature phase shift keying, orthogonal frequency divisionmultiplexing and pulse amplitude modulation. The system includes amultiplexer, the modulation block operable to demodulating a receivedfirst data stream and a second data stream received in the opticalsignal, and the multiplexer operable to multiplex the first and seconddata streams. The system includes a transmission convergence layer blockfor processing data streams received by the first transceiver, thetransmission convergence layer block operable to control thedemultiplexing of data streams including control of the multiplexer. Theoptical network is an optical distribution network. The firsttransceiver is an optical line terminator. The second and thirdtransceivers are optical network terminals or optical network units.

In general, in another aspect, the invention includes a method forprocessing data for transmission in an optical communication network,the method including: demultiplexing a data stream into a firstdemultiplexed data stream and a second demultiplexed data stream,modulating each of the first and second data streams according to anm-ary modulation format, transmitting the first modulated data streamover a first optical link; and transmitting the second modulated datastream over a second optical link.

In general, in another aspect, the invention includes an opticalcommunication system including: a demultiplexer operable to demultiplexa data stream into a first demultiplexed data stream and a seconddemultiplexed data stream, a modulation block operable to modulate eachof the first and second data streams according to an m-ary modulationformat, transmitting means operable to transmit the first modulated datastream over a first optical link and the second modulated data streamover a second optical link.

In general, in another aspect, the invention includes a method forprocessing a received optical signal in an optical communicationnetwork, the method including: equalizing a received optical signal toprovide an equalized signal, demodulating the equalized signal accordingto an m-ary modulation format to provide a demodulated signal, decodingthe demodulated signal according to an inner code to provide aninner-decoded signal, and decoding the inner-decoded signal according toan outer code.

Aspects of the invention may include one or more of the followingfeatures. The m-ary modulation format is selected from the groupconsisting of quadrature amplitude modulation, quadrature phase shiftkeying, orthogonal frequency division multiplexing and pulse amplitudemodulation. Equalizing the received optical signal includes equalizingthe received optical signal using a blind equalization routine that doesnot use a known training sequence. Equalizing the received opticalsignal includes equalizing the received optical signal using a knowntraining sequence. The known training sequence is multiplexed in a framewithin the received optical signal. The inner code includes a trelliscode. The outer code includes an error correction code. The outer codeincludes a: Reed-Solomon code; trellis code; Low-density parity-checkcode, or a Turbo code.

In general, in another aspect, the invention includes a transceiverincluding: an equalizer for equalizing a received optical signal toprovide an equalized signal, a demodulator in communication with theequalizer for demodulating the equalized signal according to an m-arymodulation format to provide a demodulated signal, an inner-decoder incommunication with the demodulator for decoding the demodulated signalaccording to an inner code to provide an inner-decoded signal, and anouter-decoder in communication with the inner-decoder for decoding theinner-decoded signal according to an outer code.

Aspects of the invention may include one or more of the followingfeatures. The transceiver includes an optical module including a firstbi-directional optical fiber interface including a first detector and afirst driver, and a second bi-directional optical fiber interfaceincluding a second detector and a second driver, and management meansfor managing data flow across the first bi-directional optical fiberinterface and across the second bi-directional optical fiber interface.The transceiver includes an optical module including a firstbi-directional optical fiber interface including a first detector and afirst driver, and a second bi-directional optical fiber interfaceincluding a second detector and a second driver, and a multiplexer formultiplexing a first demultiplexed data stream received over the firstbi-directional optical fiber interface and a second demultiplexed datastream received over the second bi-directional optical fiber interfaceinto a multiplexed data stream for transmission. The transceiverincludes an optical module including a first bi-directional opticalfiber interface including a first detector and a first driver, and asecond bi-directional optical fiber interface including a seconddetector and a second driver, and a queue manager for managing trafficfor a first bi-directional link associated with the first bi-directionaloptical fiber interface independently from traffic for a secondbi-directional link associated with the second bi-directional opticalfiber interface.

In general, in another aspect, the invention includes a transceiverincluding: an optical module including a first bi-directional opticalfiber interface including a first detector and a first driver, and asecond bi-directional optical fiber interface including a seconddetector and a second driver, and management means for managing dataflow across the first bi-directional optical fiber interface and acrossthe second bi-directional optical fiber interface.

Aspects of the invention may include one or more of the followingfeatures. The management means includes a multiplexer for multiplexing afirst demultiplexed data stream received over the first bi-directionaloptical fiber interface and a second demultiplexed data stream receivedover the second bi-directional optical fiber interface into amultiplexed data stream for transmission. The management means isconfigured to demultiplex a data stream over a plurality of fiber linksthat excludes one or more failed fiber links. The management meansincludes a queue manager for managing traffic across the firstbi-directional fiber interface independently from traffic for the secondbi-directional fiber interface. The management means is configured tochange the alignment of received data bits to adjust for an order ofoptical fiber connections to the first bi-directional optical fiberinterface and the second bi-directional optical fiber interface.

In general, in another aspect, the invention includes a method forequalizing an optical channel including: storing channel characteristicsfor the optical channel associated with a client, loading the storedchannel characteristics during a waiting period between bursts on thechannel, and equalizing a received burst from the client using theloaded channel characteristics.

Aspects of the invention may include one or more of the followingfeatures. The method includes determining that the waiting period occursbefore a burst from the client based on a schedule. The method includesupdating the stored channel characteristics. The method includesproviding a grant window, transmitting an identification number to theclient in response to receiving a serial number from the client afterthe grant window. The method includes determining a distance from anupstream device to the client. The method includes compensating forcommunication delays between the upstream device and the client based onthe determined distance.

In general, in another aspect, the invention includes a method forcommunicating data on a fiber optic network, the method including:modulating and demodulating data traffic on an optical link in thenetwork in an m-ary modulation format; encoding and decoding datatraffic on an optical link in the network according to an inner codingroutine and an outer coding routine, demultiplexing data traffic from anoptical link in the network and transmitting the data traffic across aplurality of optical fiber links in the network, multiplexing the datatraffic from the plurality of optical fiber links, and equalizing areceive channel in the network to remove temporal distortions.

Aspects of the invention may include one or more of the followingfeatures. The method includes equalizing the receive channel accordingto a blind equalization routine. The method includes equalizing thereceive channel according to a decision directed equalization routine.The method includes saving and loading coefficients for equalizing thereceive channel for each of a plurality of transmitting sources. Themethod includes conveying a training sequence for a decision directedequalization routine as part of an in-use communication protocol. Atraining sequence for a decision directed equalization routine isconveyed as part of the activation process for an optical networkterminal or optical network unit. An incorrect connection of an opticalfiber link is corrected without having to physically change theconnection.

In general, in another aspect, the invention includes a method forcommunicating on a passive optical network between a centraltransmission point and a plurality of receiving client end points, themethod including: preparing downstream data for transmission andtransmitting an optical downstream continuous mode signal demultiplexedacross a plurality of bi-directional fibers using a plurality ofwavelengths of light, receiving an optical downstream continuous modesignal demultiplexed from the plurality of bi-directional fibers usingthe plurality of wavelengths of light and recovering a downstream datatransmission, preparing upstream data for transmission and transmittingan optical upstream burst mode signal demultiplexed across the pluralityof bi-directional fibers using the plurality of wavelengths of light,and receiving an optical burst mode signal demultiplexed from theplurality of bi-directional fibers using the plurality of wavelengths oflight and recovering an upstream data transmission.

Aspects of the invention may include one or more of the followingfeatures. The central transmission point includes an optical lineterminal, and the end points are operative as transceivers in a passiveoptical network. The upstream and downstream data for transmission areconveyed by respective different industry-standard services.

Implementations of the invention may include one or more of thefollowing advantages.

A system is proposed that provides for high-speed communications overfiber optic networks. The system may include the use of the one or moreof the following techniques either individually or in combination: m-arymodulation; channel equalization; demultiplexing across multiple fibers,coding and error correction. M-ary modulation allows for increased datathroughput for a given line rate due to an increase in the number ofbits per symbol transmitted. Channel equalization reduces the effects oftemporal distortions allowing for increased reach. Demultiplexing acrossmultiple fibers allows lower lines rates for a given data throughputrate due to the increased aggregate data throughput from themultiplexing. Coding and error correction allows for a greater selectionof qualifying optical components that can be used in the network andcomplements m-ary modulation and channel equalization for overall systemperformance improvement as measured by transmit energy per bit. Thesemethods when combined (in part or in total) increase the data throughputand reach for fiber optic networks. For PONs in particular, thesemethods may increase the number of ONU/ONTs and the distance between OLTand ONU/ONT by decreasing the line rate as compared to a conventionalcommunication system of equivalent data throughput.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a fiber optic data network.

FIG. 2 illustrates a block diagram of a passive optical network.

FIG. 3 illustrates a block diagram of a high-speed communication systemfor fiber optic networks.

FIG. 4 illustrates a block diagram of an alternative high-speedcommunication system for fiber optic networks.

FIG. 5 illustrates a block diagram of an alternative high-speedcommunication system for fiber optic networks.

FIG. 6A illustrates a block diagram of an alternative high-speedcommunication system for fiber optic networks.

FIG. 6B illustrates a block diagram of an alternative high-speedcommunication system for fiber optic networks.

FIG. 7 illustrates a block diagram of an alternative high-speedcommunication system for fiber optic networks.

FIG. 8A illustrates an exemplary flow diagram for upstream burst modecommunication processing.

FIG. 8B illustrates another exemplary flow diagram for upstream burstmode communication processing.

FIG. 9 illustrates an exemplary flow diagram for a downstream continuousmode communication equalization process.

DETAILED DESCRIPTION

Referring to FIG. 1, wherein like reference numerals designate identicalor corresponding parts throughout the several views and embodiments, ahigh-level fiber optic data network 50 includes a first transceiver 100in communication with a second transceiver 101 via a fiber 108. Thefirst transceiver 100 and the second transceiver 101 include transmittercircuitry (Tx) 134, 135 to convert electrical data input signals intomodulated light signals for transmission over the fiber 108. Inaddition, the first transceiver 100 and the second transceiver 101 alsoinclude receiver circuitry (Rx) 133, 136 to convert optical signalsreceived via the fiber 108 into electrical signals and to detect andrecover encoded data and/or clock signals. First transceiver 100 andsecond transceiver 101 may contain a micro controller (not shown) and/orother communication logic and memory 131, 132 for network protocoloperation. Although the illustrated and described implementations of thetransceivers 100, 101 include communication logic and memory in a samepackage or device as the transmitter circuitry 134, 135 and receivercircuitry 133, 136, other transceiver configurations may also be used.

First transceiver 100 transmits/receives data to/from the secondtransceiver 101 in the form of modulated optical light signals of knownwavelength via the optical fiber 108. The transmission mode of the datasent over the optical fiber 108 may be continuous, burst or both burstand continuous modes. Both transceivers 100,101 may transmit a samewavelength (e.g., the light signals are polarized and the polarizationof light transmitted from one of the transceivers is perpendicular tothe polarization of the light transmitted by the other transceiver).Alternatively, a single wavelength can be used by both transceivers 100,101 (e.g., the transmissions can be made in accordance with atime-division multiplexing scheme or similar protocol).

In another implementation, wavelength-division multiplexing (WDM) mayalso be used. WDM is herein defined as any technique by which twooptical signals having different wavelengths may be simultaneouslytransmitted bi-directionally with one wavelength used in each directionover a single fiber. In yet another implementation, coarsewavelength-division multiplexing (CWDM) or dense wavelength-divisionmultiplexing (DWDM) may be used. CWDM and DWDM are herein defined as anytechnique by which two or more optical signals having differentwavelengths are simultaneously transmitted in the same direction. Thedifference between CWDM and DWDM is CWDM wavelengths are typicallyspaced 20 nanometers (nm) apart, compared with 0.4 nm spacing for DWDMwavelengths. Both CWDM and DWDM may be used in bi-directionalcommunications. In bi-directional communications, e.g. ifwavelength-division multiplexing (WDM) is used, the first transceiver100 may transmit data to the second transceiver 101 utilizing a firstwavelength of modulated light conveyed via the fiber 108 and, similarly,the second transceiver 101 may transmit data via the same fiber 108 tothe first transceiver 100 utilizing a second wavelength of modulatedlight conveyed via the same fiber 108. Because only a single fiber isused, this type of transmission system is commonly referred to as abi-directional transmission system. Although the fiber optic networkillustrated in FIG. 1 includes a first transceiver 100 in communicationwith a second transceiver 101 via a single fiber 108, otherimplementations of fiber optic networks, such as those having a firsttransceiver in communication with a plurality of transceivers via aplurality of fibers (e.g. shown in FIG. 2), may also be used.

Electrical data input signals (Data IN 1) 115, as well as any optionalclock signal (Data Clock IN 1) 116, are routed to the transceiver 100from an external data source (not shown) for processing by thecommunication logic and memory 131. Communication logic and memory 131process the data and clock signals in accordance with an in-use networkprotocol. Communication logic and memory 131,132 provides managementfunctions for received and transmitted data including queue management(e.g., independent link control) for each respective link,demultiplexing/multiplexing and other functions as described furtherbelow. The processed signals are transmitted by the transmittercircuitry 134. The resulting modulated light signals produced from thefirst transceiver's 100 transmitter 134 are then conveyed to the secondtransceiver 101 via the fiber 108. The second transceiver 101, in turn,receives the modulated light signals via the receiver circuitry 136,converts the light signals to electrical signals, processes theelectrical signals using the communication logic and memory 132 (inaccordance with an in-use network protocol) and, optionally, outputs theelectrical data output signals (Data Out 1) 119, as well as optionalclock signals (Data Clock Out 1) 120.

Similarly, the second transceiver 101 receives electrical data inputsignals (Data IN 1) 123, as well as any optional clock signals (DataClock IN) 124, from an external data source (not shown) for processingby the communication logic and memory 132 and transmission by thetransmitter circuitry 135. The resulting modulated light signalsproduced from the second transceiver's 101 transmitter 135 are thenconveyed to the first transceiver 100 using the optical fiber 108. Thefirst transceiver 100, in turn, receives the modulated light signals viathe receiver circuitry 133, converts the light signals to electricalsignals, processes the electrical signals using the communication logicand memory 131 (in accordance with an in-use network protocol), and,optionally, outputs the electrical data output signals (Data Out 1) 127,as well as any optional clock signals (Data Clock Out 1) 128.

Fiber optic data network 50 may also include a plurality of electricalinput and clock input signals, denoted herein as Data IN N 117/125 andData Clock IN N 118/126, respectively, and a plurality of electricaloutput and clock output signals, denoted herein as Data Out N 129/121and Data Clock Out N 130/122, respectively. The information provided bythe plurality of electrical input signals may or may not be used by agiven transceiver to transmit information via the fiber 108 and,likewise, the information received via the fiber 108 by a giventransceiver may or may not be outputted by the plurality of electricaloutput signals. The plurality of electrical signals denoted above can becombined to form data plane or control plane bus(es) for input andoutput signals respectively. In some implementations, the plurality ofelectrical data input signals and electrical data output signals areused by logic devices or other devices located outside (not shown) agiven transceiver to communicate with the transceiver's communicationlogic and memory 131, 132, transmit circuitry 134, 135, and/or receivecircuitry 133,136.

FIG. 2 illustrates an implementation of a passive optical network (PON)52, where the functions described above associated with the firsttransceiver 100 and the second transceiver 101 of FIG. 1, areimplemented in an optical line terminator (OLT) 150 and one ore moreoptical networking units (ONU) 155, and/or optical networking terminals(ONT) 160, respectively. PON(s) 52 may be configured in either apoint-to-point network architecture, wherein one OLT 150 is connected toone ONT 160 or ONU 155, or a point-to-multipoint network architecture,wherein one OLT 150 is connected to a plurality of ONT(s) 160 and/orONU(s) 155. In the implementation shown in FIG. 2, an OLT 150 is incommunication with multiple ONTs/ONUs 160, 155 via a plurality ofoptical fibers 152. The fiber 152 coupling the OLT 150 to the PON 52 isalso coupled to other fibers 152 connecting the ONTs/ONUs 160, 155 byone or more passive optical splitters 157. All of the optical elementsbetween an OLT and ONTs/ONUs are often referred to as the OpticalDistribution Network (ODN). Other alternate network configurations,including alternate implementations of point-to-multipoint networks arealso possible.

FIG. 3 shows a system block diagram for an implementation of transceiver100. It will be appreciated that, while not always shown, one or moreelements or blocks in the following embodiments may be sealed in one ormore faraday cages or combined with blocks in faraday cages alreadyshown. It will also be appreciated that, while not shown, one or moreelements or blocks in the following embodiments may be combined onto oneor more integrated circuits (IC) or surface mount photonic (SMP)devices. The following is a description of the functions andresponsibilities that are part of an implementation of the CommunicationLogic & Memory 131 of transceiver 100. The Communication Logic & Memory131 includes an asynchronous or synchronous system transmit (TX)interface 301 and receive (RX) interface 302 that is supported by the TXPath 303 and RX Path 304 blocks. System interfaces 301,302 andmanagement or control interfaces can be selected from conventionalinterfaces including serial, serial XFI, parallel, GMII, XGMII, SGMII,RGMII or XAUI or some other interface may be used. TX Path 303 and RXPath 304 blocks manage the TX and RX interfaces 301,302 and feed datainto and get data from the transmission convergence layer or mediaaccess control (TC-Layer/MAC) block 305. TX Path 303 and RX Path 304blocks may perform line code adaptation functions (e.g., line codingused outside the transceiver can be terminated by a TX Path block 303 orsourced by a RX Path block 304 to allow a bit stream, cell, frame,and/or packet formatted data to be adapted for processing by aTC-Layer/MAC block 305). The TC-Layer/MAC 305 block creates thetransport system that the data traffic, management and control agentswill exploit. TC-Layer/MAC 305 block includes a TC-layer protocol stacksuch as specified in the ITU G.984 specification (incorporated herein byreference), IEEE 802.3ah MAC protocol stack specification (incorporatedherein by reference) or a derivative thereof. A variety of otherprotocol stacks may also be used. The TC-Layer/MAC 305 block may performthe additional functions of equalizer, coding, queue and demultiplexingmanagement. The TC-Layer/MAC 305 block sends transmit data to a DeMux306 block, which splits the transmitting data into a plurality of datapaths (two paths shown in FIG. 3) for demultiplexing data acrossmultiple fibers. Some implementations need not include DeMux 306 block(and hence do not support demultiplexing data across multiple fibers).DeMux 306 block may demultiplex data across a subset of fibers toexclude fibers experiencing link failure to ensure data communicationscontinue. The exclusion of fiber links experiencing failure iscontrolled by the TC-Layer/MAC 305 block as part of the demultiplexingmanagement function.

After DeMux 306 block, in one implementation, the transmit paths haveanalogous processing blocks. In an alternative implementation,independent signal processing can be supported in each path. FIG. 3shows two transmit paths, though more can be included. In a transmitpath, the transmit data is provided to the outer coder 307 a, 307 bblock. In one implementation, outer coder 307 a performs a reed-solomoncoding. The outer coder 307 a, 307 b block provides data to the innercoder 308 a, 308 b block. In order to improve the energy per bitrequired to deliver the transmitting data, an inner coder 308 a, 308 bis used. Outer coder 307 a, 307 b may be used to support forward errorcorrection (FEC) recovery of bit(s) errors. In one implementation, innercoder 308 a, 308 b implements a trellis coding method. Data from theinner coder 308 a, 308 b is provided to Modulation (MOD) 309 a, 309 bblock. Alternatively, in some implementations, the outer coder 307 a,307 b and inner coder 308 a, 308 b blocks are not used, and the outputof the DeMux 306 block is provided directly to the MOD 309 a, 309 bblock. Other outer coding methods that work on bit or symbol streams ofarbitrary length can be used, for example linear block codes such asLow-density parity-check (LDPC) and convolutional codes such as Turbocode may be used. Other inner coding methods that are complementary tothe outer code as well as inner coding methods that are designed toshape or control the relative intensity noise (RIN) of the opticaltransmitter to improve overall system performance may be used. Forexample, an inner coder that dynamically adapts to measured RIN orcompensates for measured temperature or other artifacts of laser designmay be used.

To increase the number of bits per symbol transmitted, m-ary modulationis performed in the MOD 309 a, 309 b block. In one implementation, anm-ary modulation method such as Quadrature Amplitude Modulation (QAM),QAM-32, QAM-256, Pulse Amplitude Modulation (PAM), PAM-5, PAM-17,Quadrature Phase Shift Keying (QPSK), differential QPSK (DQPSK),return-to-zero QPSK (RZ-QPSK), dual-polarized QPSK (DP-QPSK), orOrthogonal Frequency Division Multiplexing (OFDM) is used. Other m-arymodulation communication methods can be used, in particular othercoherent modulation techniques which are known in the art. Afterprocessing by the MOD 309 a, 309 b block, the transmit data is convertedto an analog signal by a Digital to Analog Converter (DAC) 310 a, 310 b.In one implementation, DAC 310 a, 310 b is configured to shape,condition or emphasize the signal for improved transmission performance.The DAC 310 a, 310 b passes the transmit data via electrical signals 311a, 311 b to the laser driver (Driver) 312 a, 312 b as part of animplementation of TX 134 in an Optical Module 326. The driver 312 a, 312b drives an optical transmitter, such as the Laser Diode (LD) 313 a, 313b, which transmits light in response to transmit data signals receivedfrom the driver 312 a, 312 b. The light emitted from LD 313 a, 313 b isdirected into the fibers 314 a, 314 b with the aid of a fiber opticinterface (not shown). The fiber optic interface may include thenecessary components (e.g., filters) to implement WDM, CWDM or DWDMfunctions.

On the receive side of the transceiver 100 as part of an implementationof RX 133 in an Optical Module 326, light propagated across an ODN (notshown in FIG. 3) travels over fibers 314 a, 314 b through a fiber opticinterface (not shown) and is received by an optical detector, such asthe photo diode (PD) 315 a, 315 b. In response, the PD 315 a, 315 bprovides a photocurrent to the TransImpedance Amplifier (TIA) 316 a, 316b that converts the photocurrent into an electrical voltage signal. Theelectrical voltage signal from the TIA 316 a, 316 b is then transmittedto a Linear Amplifier (LA) 317 a, 317 b as a differential signal or asingle-ended signal 318 a, 318 b. The LA 317 a, 317 b performs signalconditioning on the received electrical voltage signal to provideincreased resolution and system performance. The LA 317 a, 317 bprovides an electrical signal 319 a, 319 b to a Clock Data Recovery(CDR) and Equalization (EQ) 320 a, 320 b block that recovers clock anddata signals and performs equalization on the received data, which isthen provided to a De-Mod & Inner Decoder 323 a, 323 b. The CDR & EQ 320a, 320 b block may implement a blind equalization method ordecision-directed equalization method. Blind equalization is discussedfurther below. Other equalization methods may be used, particularlythose that aid the CDR. The De-Mod & Inner Decoder 323 a, 323 b blockperforms complementary de-modulation to the m-ary modulation performedin the MOD 309 a, 309 b block as well as a complementary decoding methodto the coding method performed in the Inner Coder 308 a, 308 b block. Inone implementation, De-Mod & Inner Decoder 323 a, 323 b includes aViterbi decoder. Other decoding means may be used. Received data is thenprovided to the outer decoder 324 a, 324 b block, which performs acomplementary decode to the error detection and/or recovery methodchosen in the outer coder 307 a, 307 b block. After demodulation anddecoding, the received data is then provided to the Mux 325 block thatperforms a complementary function to the DeMux 306 block. The combinedreceived data is then provided to the TC-Layer/MAC 305. Inimplementations without Outer Coder 307 a, 307 b and Inner Coder 308 a,308 b blocks, the output of the CDR & EQ 320 a, 320 b block is provideddirectly to the Mux 325 block.

The RX 133,136 and TX 134,135 circuitry of transceivers 100,101, orportions thereof, for example, PD 315 a, 315 b and LA 317 a, 317 b, canbe combined within industry standard optical modules. Common opticalmodule standards are 300 pin, XENPAK, X2, and XPAK transponders and XFPor SFP and SFP+ transceivers. These optical modules includeunidirectional fiber links with one fiber link for transmit path and asecond fiber link for the receive path. However, implementations ofoptical modules 326, 401, 501 incorporate a plurality of bi-directionalfiber links for transmitting demultiplexed data on separate fiber links.Any of a variety of optical couplers may be used to separate and/orcombine light propagating into or out of the fiber links. These opticalmodules 326, 401, 501 used herein can conform to a form factor ofstandard optical modules such as the 300 pin, XENPAK, X2, XPAK, XFP orSFP and SFP+. Other form factors may also be used.

Alternatively, in other implementations of transceiver 100, functionsdescribed above may be integrated into various different components. Forexample, in the implementation of transceiver 100 shown in FIG. 4,various functions may be incorporated into optical module 401 such as:digital to analog conversion 310 a, 310 b; analog to digital conversion321 a, 321 b; clock data recovery and equalization 320 a, 320 b; m-arymodulation 309 a, 309 b; m-ary de-modulation 323 a, 323 b; inner coder308 a, 308 b; inner de-coder 323 a, 323 b; outer coder 307 a, 307 b;outer de-coder 324 a, 324 b, and the De-Mux 306 and Mux 325 functionsthat enable demultiplexing across multiple fibers. The optical module401 may have an interface that can connect to existing TC-Layer or MACimplementations currently produced. In another alternativeimplementation the digital to analog conversion 310 a, 310 b; analog todigital conversion 321 a, 321 b, and the clock data recovery 320 a, 320b functions are incorporated into an optical module (not shown). In yetanother alternative implementation of the transceiver 100 as shown inFIG. 5, an optical module 501 includes the De-Mux 306 and Mux 325functions enabling demultiplexing across multiple fibers. The opticalmodule 501 may have an interface that can connect to existing TC-Layeror MAC implementations currently produced.

Alternative implementations of transceiver 100 utilizing a single fiberlink 314 a (without demultiplexing across multiple fibers) areillustrated in FIG. 6A-6B. Alternatively, an implementation of thetransceiver 100 may utilize multiple fiber links 314 a, 314 b while notperforming demultiplexing across multiple fibers, as illustrated in FIG.7. In this implementation, the TC-Layer/MAC 701 block manages the fiberlinks as independent fiber links that all connect to the same endpoint(s) on the network. In one implementation, TC-Layer/MAC 701 blockis a derivative of the Transmission Convergence Layer specified in ITUG.984 or MAC specified in IEEE 802.3ah, with the added functionality ofqueue management of the traffic across the plurality of independentfiber links. The TC-Layer/MAC 701 block may exclude use of one or morefiber links if the fiber link experiences a failure. This exclusion offailed fiber links enables the TC-Layer/MAC 701 block (i.e., queuemanagement function) to continue providing service across a PON usingthe remaining active links. Each fiber can be deployed across physicallydifferent paths to provide optical fiber distribution path diversity andimproved protection against failures. Failures may originate in theoptical module or across elements of the ODN such as fiber or connectorbreaks.

Channel Equalization

An implementation for a channel equalization routine executed in the CDR& EQ 320 a, 320 b block includes determining coefficient settings orweights that are applied to the received data to remove undesiredinformation (e.g. intersymbol interference (ISI)) or noise from thereceived data and thereby increase the sensitivity, dynamic range ofdetecting signals and accuracy of receiving signals. Channelequalization can include a training or convergence period in whichcharacteristics of the channel are learned or accounted for andcoefficients, filter variables, or weights are adapted before or whileprocessing the received data. Decision-directed equalization is anequalization method in which a known training sequence is sent duringthe training period and the receiver/transceiver uses the knowledge ofthe training sequence to learn about the channel characteristics. Thetraining sequence can be multiplexed within a PON's TC-Layer framingprotocol. Blind equalization is a process during which an unknown inputdata sequence is recovered from the output signal of an unknown channel(i.e., current equalization data for a given channel is unknown orotherwise unavailable). Other equalization methods may be used, digitalsignal processing methods, or methods that improve the accuracy ofprocessing received data signals or improve the efficiency of processingreceived data signals (e.g., reducing data acquisition time, reducingpower consumed) by saving or storing a first set of settings generatedby processing data from a first ONU/ONT and then load previously savedsecond set of settings previously generated by processing data from asecond ONU/ONT before processing another set of data from the secondONU/ONT.

One mode of communications used by a PON, e.g., for upstream datatraffic (ONU/ONT to OLT direction), is “burst mode” communications. Forexample, upstream communications on a PON may include a link sharedamong multiple clients or ONUs/ONTs via time division multiplexing undercontrol by an OLT. The upstream direction is divided into time slots;each time slot includes a defined number of bits. A given ONU/ONT isgranted some number of time slots during which to transmit an upstreamframe of data to an OLT. The upstream direction uses an orchestratedcollection of bursts from the different ONU/ONTs, coordinated by the OLTthat tries to maximize upstream traffic bandwidth efficiency byminimizing empty slots.

A flow chart for an exemplary upstream burst mode communicationequalization process is shown in FIG. 8A. To read or interpret theupstream data traffic from a client ONU/ONT, an OLT trains and/orequalizes the channel for that client ONU/ONT. Since the ONU/ONTs may beat different distances from the OLT and all do not share the same fiber,different channel characteristics result. Communication efficiencies maybe obtained by determining 800 a set of equalization coefficients for achannel during a burst from a client, saving 801 the determinedequalization coefficients, entering a wait period 802 (also known as aPON's silence period when no client ONU/ONTs are transmitting upstream),and loading 803 the stored equalization coefficients before a next burstfrom the client (during the wait period), to avoid re-training orre-equalizing on every burst communication. The OLT has prior knowledgeof which ONU/ONT will be transmitting data during which time slots andcan use this knowledge during the time between burst communications(during the wait period) to load 803 an appropriate set of coefficientspertaining to the particular ONU/ONT transmitting prior to receiving 804its next upstream burst. This process continues for subsequent bursts.In one implementation, periodic (though not coincident with eachcommunication burst) updates to the channel characteristics may be made(and stored). The OLT can save 801 coefficients that have converged orhave been trained after receiving burst communications from the firstONU/ONT and load 803 a new set of coefficients during the wait periodbetween bursts (i.e., before an incoming upstream burst from a secondONU/ONT). In another exemplary implementation, the OLT can save or store801 coefficients or settings during the wait period 802 between burst asshown in FIG. 8B. In one implementation, in addition to or alternativeto storage of coefficient data, the OLT may also save and load innerand/or outer coding states between bursts improving the efficiency ofcommunication, similar to the equalization process of FIG. 8A-8B. Othermethods that improve the accuracy and efficiency of processing burstmode data from specific ONUs/ONTs may be used following a similarprocess.

Another mode of communications used by a PON, e.g., for downstream datatraffic (OLT to ONU/ONT direction), is “continuous mode” communications.In one implementation, a receiver, such as an ONU/ONT, equalizes areceived data channel using either one of a blind equalization or adecision directed equalization method.

A flow chart for an exemplary PON activation process is shown in FIG. 9.In a PON in which a decision directed method is used for training anONU/ONT receiver, a continuous mode transmitter, such as an OLTtransmitter, sends a training sequence 900 multiplexed within a PON'sTC-Layer downstream frame protocol. In a PON in which a blindequalization method is used, the OLT needed not send this trainingsequence 900. An ONU/ONT equalizes its received downstream channel 901before it is able to receive and interpret PON network parameters 902.If the OLT has not been previously informed of the existence of theONU/ONT then the ONU/ONT awaits an upstream grant window 903 availablefor new ONU/ONTs to respond to the OLT with its serial number 904. Afterthe ONU/ONT has received an upstream grant window and processed PONsystem parameters, the ONU/ONT sends a training sequence 905 and thenits serial number 904 to the OLT. In a PON in which blind equalizationis used the ONU/ONT need not send a training sequence 905. After the OLThas received the ONU/ONT serial number the OLT will assign and send theONU/ONT an identification number. If the ONU/ONT does not receive anidentification number 906 a, the ONU/ONT returns to waiting for anupstream grant window for new ONU/ONTs 903. Once the ONU/ONT receives anidentification number 906 b, the OLT performs ranging 907 to determinethe distance between the OLT and ONU/ONT and then compensates for thecommunication timing delays. The ONU/ONT can perform updatescontinuously or periodically depending on the equalization methodemployed. After the downstream continuous mode channel and the upstreamburst mode channel have been equalized, both ends of the PONtransmission link are equalized and the ONU/ONT enters its normaloperating state 908.

Link Connection Errors

A system has been proposed that includes demultiplexing across multiplefibers as is shown above with reference to FIGS. 3-6. In systems usingdemultiplexing across multiple fibers, fibers can be connectedincorrectly at installation. For example, a first transceiver 100, suchas is shown in FIG. 3, with fibers 314 a and 314 b can be connected to asecond transceiver 101 with fiber 314 b connected in place of fiber 314a, and fiber 314 a connected in place of fiber 314 b. The incorrectconnection in this example may cause the first and second transceiversto not establish communications due to misalignment of bits duringmultiplexing of received data.

Information in a frame is used to synchronize a receiver (e.g.,transceiver 101) with the beginning of a frame (e.g., a “framedelimiter”). The process of discovering the beginning of a frame iscalled “frame synchronization.” In specific protocols such as G.984, thedownstream frame delimiter is called Psync, the upstream frame delimiteris called Delimiter and the process of frame synchronization in thedownstream is called the HUNT. In one implementation, TC-Layer/MAC 305block performs frame synchronization. In one implementation, specificbit patterns or values for frame delimiters are used that are unique foreach fiber to differentiate one fiber from another or the order of fiberconnections to correctly multiplex received data. The use of uniqueframe delimiters allows the TC-Layer/MAC 305 block to change thealignment of received data bits during multiplexing to adjust for theorder of the fiber connections, without having to physically change theconnections. Management of the bit alignment in this implementationforms part of the TC-Layer/MAC's 305 block demultiplexing managementresponsibilities and functions.

Alternatively, the TC-Layer/MAC 305 block may assume an order for thefiber connections to determine the alignment of bits for multiplexingthe received data and attempt frame synchronization. After a period oftime with no frame synchronization success, the TC-Layer/MAC 305 blockmay assume a different order for the fiber connections and change thealignment of bits during multiplexing and attempt frame synchronizationagain. The process may repeat, including changing the alignment of bitsto reflect other configurations during the multiplexing, and framesynchronization attempts continue until frame synchronization succeeds.In yet another alternative implementation, the TC-Layer/MAC 305 blockmay assume and attempt frame synchronization on all possiblecombinations of bit alignments in parallel, one of which will succeed inachieving frame synchronization.

Although the invention has been described in terms of particularimplementations, one of ordinary skill in the art, in light of thisteaching, can generate additional implementations and modificationswithout departing from the spirit of or exceeding the scope of theclaimed invention. Accordingly, it is to be understood that the drawingsand descriptions herein are proffered by way of example to facilitatecomprehension of the invention and should not be construed to limit thescope thereof.

What is claimed is:
 1. A pluggable optical transceiver module comprisingof: an electrical system interface for receiving a first electrical datasignal and for transmitting a second electrical data signal; an m-arymodulator for increasing the number of bits per symbol in the firstelectrical data signal to produce a first m-ary modulation signal; adigital to analog converter for converting the first m-ary modulationsignal to a first electrical m-ary analog modulation signal; a driverfor amplifying the first electrical m-ary analog modulation signal to anamplified first electrical m-ary analog modulation signal to drive anoptical transmitter; the optical transmitter for emitting a firstoptical signal on a first wavelength responsive to and representative ofthe amplified first electrical m-ary analog modulation signal from thedriver; an optical detector for receiving a second optical signal on asecond wavelength to produce an electrical analog signal; an amplifierfor amplifying the electrical analog signal to an amplified electricalanalog signal; a clock data recovery unit for recovering clock and datainformation to produce a second m-ary modulation signal from theamplified electrical analog signal; an equalizer for performingequalization on the second m-ary modulation signal to remove noise; andan m-ary demodulator for decreasing the number of bits per symbol in theequalized second m-ary modulation signal to produce the secondelectrical data signal.
 2. The pluggable optical transceiver module ofclaim 1, wherein the form factor of the pluggable optical transceivermodule is a pluggable form factor standard.
 3. The pluggable opticaltransceiver module of claim 2, wherein the pluggable form factorstandard of the pluggable optical transceiver module is a small formfactor pluggable (SFP) or SFP+ form factor standard.
 4. The pluggableoptical transceiver module of claim 2, wherein the pluggable form factorstandard of the pluggable optical transceiver module is an XFP formfactor standard.
 5. The pluggable optical transceiver module of claim 1,wherein the m-ary modulator and the m-ary demodulator perform m-aryquadrature amplitude modulation and de-modulation, respectively, thatuses two or more bits per symbol.
 6. The pluggable optical transceivermodule of claim 1, wherein the m-ary modulator and the m-ary demodulatorperform m-ary quadrature phase shift keying modulation andde-modulation, respectively, that uses two or more bits per symbol. 7.The pluggable optical transceiver module of claim 1, wherein the m-arymodulator and the m-ary demodulator perform orthogonal frequencydivision multiplexing modulation and de-modulation, respectively, thatuses two or more bits per symbol.
 8. The pluggable optical transceivermodule of claim 1, wherein the electrical system interface includesparallel signals.
 9. The pluggable optical transceiver module of claim1, wherein the electrical system interface includes serial signals. 10.The pluggable optical transceiver module of claim 1, wherein the firstelectrical data signal and second electrical data signal includesparallel electrical data signals.
 11. The pluggable optical transceivermodule of claim 1, wherein the electrical system interface includes anattachment unit interface (AUI) based interface.
 12. The pluggableoptical transceiver module of claim 1, wherein the first optical signalon a first wavelength is configured to emit into a first optical fiberand the second optical signal on a second wavelength is configured to bereceived from the first optical fiber.
 13. The pluggable opticaltransceiver module of claim 1, wherein the first optical signal on afirst wavelength is configured to emit into a first optical fiber andthe second optical signal on a second wavelength is configured to bereceived from a second optical fiber.
 14. The pluggable opticaltransceiver module of claim 1, wherein the noise removed by theequalizer includes intersymbol interference (ISI).
 15. The pluggableoptical transceiver module of claim 1, wherein the equalizer performsdecision directed equalization on the second m-ary modulation signal toremove noise.
 16. The pluggable optical transceiver module of claim 1,wherein the equalizer performs blind equalization on the second m-arymodulation signal to remove noise.
 17. The pluggable optical transceivermodule of claim 1, wherein the digital to analog converter is configuredto shape, condition, or emphasize the first electrical m-ary analogmodulation signal for improved transmission performance.
 18. Thepluggable optical transceiver module of claim 1, wherein the electricalsystem interface includes receiving a third electrical data signal andtransmitting a fourth electrical data signal, and the pluggable opticaltransceiver module further includes: a second m-ary modulator forincreasing the number of bits per symbol in the third electrical datasignal to produce a third m-ary modulation signal; a second digital toanalog converter for converting the third m-ary modulation signal to asecond electrical m-ary analog modulation signal; a second driver foramplifying the second electrical m-ary analog modulation signal to anamplified second electrical m-ary analog modulation signal to drive asecond optical transmitter; the second optical transmitter for emittinga third optical signal on a third wavelength responsive to andrepresentative of the amplified second electrical m-ary analogmodulation signal from the second driver; a second optical detector forreceiving a fourth optical signal on a fourth wavelength to produce asecond electrical analog signal; a second amplifier for amplifying thesecond electrical analog signal to an amplified second electrical analogsignal; a second clock data recovery unit for recovering clock and datainformation to produce a fourth m-ary modulation signal from theamplified second electrical analog signal; a second equalizer forperforming equalization on the fourth m-ary modulation signal to removenoise; and a second m-ary demodulator for decreasing the number of bitsper symbol in the equalized fourth m-ary modulation signal to producethe fourth electrical data signal.
 19. The pluggable optical transceivermodule of claim 18, wherein the third optical signal on a thirdwavelength is configured to emit into a first optical fiber and thefourth optical signal on a fourth wavelength is configured to bereceived from the first optical fiber.
 20. The pluggable opticaltransceiver module of claim 18, wherein the third optical signal on athird wavelength is configured to emit into a first optical fiber andthe fourth optical signal on a fourth wavelength is configured to bereceived from a second optical fiber.
 21. The pluggable opticaltransceiver module of claim 18, wherein the third electrical data signaland fourth electrical data signal includes parallel electrical datasignals.
 22. A pluggable optical transceiver module comprising of: anelectrical system interface for receiving a first electrical data signaland for transmitting a second electrical data signal; an m-ary modulatorfor increasing the number of bits per symbol in the first electricaldata signal to produce a first m-ary modulation signal; a digital toanalog converter for converting the first m-ary modulation signal to afirst electrical m-ary analog modulation signal; a driver for amplifyingthe first electrical m-ary analog modulation signal to an amplifiedfirst electrical m-ary analog modulation signal to drive an opticaltransmitter; the optical transmitter for emitting a first optical signalon a first wavelength responsive to and representative of the amplifiedfirst electrical m-ary analog modulation signal from the driver; anoptical detector for receiving a second optical signal on a secondwavelength to produce an electrical analog signal; an amplifier foramplifying the electrical analog signal to an amplified electricalanalog signal; a clock data recovery unit for recovering clock and datainformation to produce a second m-ary modulation signal from theamplified electrical analog signal; an equalizer for performingequalization on the second m-ary modulation signal to remove noise; anm-ary demodulator for decreasing the number of bits per symbol in theequalized second m-ary modulation signal to produce the secondelectrical data signal; and wherein the m-ary modulator and the m-arydemodulator perform m-ary pulse amplitude modulation and de-modulation,respectively, that uses two or more bits per symbol.
 23. A pluggableoptical transceiver module comprising of: an electrical system interfacefor receiving a first electrical data signal and for transmitting asecond electrical data signal; an m-ary modulator for increasing thenumber of bits per symbol in the first electrical data signal to producea first m-ary modulation signal; a digital to analog converter forconverting the first m-ary modulation signal to a first electrical m-aryanalog modulation signal; a driver for amplifying the first electricalm-ary analog modulation signal to an amplified first electrical m-aryanalog modulation signal to drive an optical transmitter; the opticaltransmitter for emitting a first optical signal on a first wavelengthresponsive to and representative of the amplified first electrical m-aryanalog modulation signal from the driver; an optical detector forreceiving a second optical signal on a second wavelength to produce anelectrical analog signal; an amplifier for amplifying the electricalanalog signal to an amplified electrical analog signal; a clock datarecovery unit for recovering clock and data information to produce asecond m-ary modulation signal from the amplified electrical analogsignal; an equalizer for performing equalization on the second m-arymodulation signal to remove noise; an m-ary demodulator for decreasingthe number of bits per symbol in the equalized second m-ary modulationsignal to produce the second electrical data signal; and wherein theequalizer includes coefficients or weights configured to remove noisefrom the second m-ary modulation signal.